EEG region of interest(ROI)

I have EEG data collected from 32 channel electrode. I measured EEG across an entire condition. Then segmented each condition in one-second segments, then fast Fourier transformation. So I am looking at the general activity of the brain during task performance, and not event-related.

I however want to examine differences in frequency bands across the parietal regions across 3 different conditions. I have computed the average frequency power bands i.e., alpha, theta, gamma, etc for all electrodes. I am wondering how to compute the average power for my region of interest, for example, can I add up the frequency power spectrum for the electrodes in the region I am interested in? eg, the alpha spectrum for left parietal region P3 + P7 ) or find the average (eg, the alpha spectrum (P3 + P7)/2 ). Are there better ways to do that?

Also does the correlation between electrodes represent coherence across region?